1. Field of the Invention
The present invention relates to the field of test specimens for evaluating crack propagation due to corrosion. More specifically, the invention relates to a unique apparatus and method for testing the effects of a corrodant upon a structure which has a support base and a cracked cladding layer deposited thereto.
2. Description of the Prior Art
The problem of excessive personnel exposures caused by high background radiation levels in a nuclear reactor primary system, such as in pressurized water reactor (PWR) systems, and the resultant economic cost of requiring personnel rotation to minimize individual exposure is significant at many nuclear plants. These background levels are principally due to the buildup of corrosion products in certain areas of the plant. The buildup of corrosion products exposes workers to high radiation levels during routine maintenance and refueling outages. The long term prognosis is that personnel exposure levels will continue to increase.
As a nuclear power plant operates, the surfaces in the core and primary system corrode. Corrosion products, referred to as crud, are activated by transport of the corroded material to the core region by the reactor coolant system (RCS). Subsequent release of the activated crud and redeposition elsewhere in the system produces radiation fields in piping and components throughout the primary system, thus increasing radiation levels throughout the plant. The activity of the corrosion product deposits is predominately due to Cobalt 58 and Cobalt 60. It is estimated that 80-90% of personnel radiation exposure can be attributed to these elements.
One way of controlling worker exposure, and of dealing with this problematic situation, is to periodically decontaminate the nuclear steam supply system using chemicals, thereby removing a significant fraction of the corrosion product oxide films. Prior techniques had done very little to decontaminate the primary system as a whole, typically focusing only on the heat exchanger (steam generator) channel heads.
Two different chemical processes, referred to as LOMI (developed in England under a joint program by EPRI and the Central Electricity Generating Board) and CAN-DEREM (developed by Atomic Energy of Canada, Ltd.), have been used for small scale decontamination in the past. These processes are multi-step operations, in which various chemicals are injected, recirculated, and then removed by ion-exchange. Although the chemicals are designed to dissolve the corrosion products, some particulates are also generated. While these chemical processes had typically been used on only a localized basis, use of these chemical processes has now been considered for possible application on a large scale, full system chemical decontamination as set forth in U.S. Pat. No. 5,089,216, entitled "System For Chemical Decontamination Of Nuclear Reactor Primary Systems", and incorporated herein by reference.
One phase in the development of satisfactory dilute chemical decontamination (DCD) systems, such as CAN-DEREM and LOMI processes, is the study of the corrosive effects of DCD solutions on the materials and components in the RCS. There are components in the RCS that consist of carbon steel cladded with stainless steel on the exposed side. Thermal and mechanical stresses may cause small cracks to develop in the stainless steel cladding of these components.
Since it is known that these cracks exist, it is necessary to determine the corrosive effects of the DCD solutions which might penetrate through the cracks during the decontamination process. If the DCD solutions are corrosive in nature, they may impair the integrity of the underlying carbon steel.
Prior art testing specimens for studying the crack propagation in structures usually employed a bar type specimen. This specimen was rectangular in shape with a carbon steel base and a stainless steel surface. A crack was then cut into the stainless steel surface and the propagation of the crack was studied as stresses were applied to the surfaces. Such a specimen is ineffective to study the effects of corrodant penetration and subsequent possible crack propagation mainly because the crack extended to the edge of the test specimen and the sides of the test specimen were exposed to the corrodant. This condition would not occur in piping structures. Also, the crack cut into the stainless steel surface was much wider than an actual hairline crack which actually exists inside a piping structure located along the RCS. Therefore, a need exists to develop a testing specimen which could more accurately simulate the environment found in such cracked cladding systems.